As is well-known in the art, when shifting of an automatic transmission for a vehicle is executed, in order to prevent a delay in the shift and a shift shock, it is necessary to control hydraulic pressure supplied to the frictional engagement means such as a clutch and a brake. Therefore, control systems have been developed which are so constructed that the engaging pressure acting on one of the two frictional engagement means required to switch between engaged and released states is directly controlled by means of a pressure modulating valve.
For instance, in an automatic transmission equipped with a gear train wherein one brake is engaged in the forward second gear stage and the brake is released with another brake engaged in up-shifting to the forward third gear stage, gear shifting between these gear stages becomes so-called clutch-to-clutch shift. It is sometimes necessitated to control the hydraulic pressure for one of the two brake (for example a brake to set the forward second speed gear stage for instance) by means of a pressure control valve adjustable in pressure modulating level.
Although a signal pressure for setting the pressure modulating level of the pressure control valve could be applied by a linear solenoid valve specially devised for the control valve, inconvenience of large-sized control system will be caused due to increased number of solenoid valves in such a structure.
In order to overcome the above-mentioned inconvenience, application of a conventional linear solenoid valve as a measure for changing the pressure modulating level of the above-mentioned control valve can be considered. In this case, since the linear solenoid valve is to be operated for setting a pressure modulating level of the above-mentioned control valve when required while being operated for its primary purpose on the other hand, the linear solenoid valve is necessarily so constructed as to switch its output pressure (signal pressure) between primary control object and the control valve, making it general to use a switching valve; wherein, in addition, the conventional linear solenoid valve can be used both for its primary purposes and as the switching valve.
In addition, it can be considered to take measures against pressure modulation failure; for example, to apply the line pressure provided through a shifting valve directly to the brake in a case of a failure where adjustment of braking pressure for the second gear stage by means of the control valve is impossible, thereby making the setting of the second gear stage sure.
When the control system is composed as mentioned above, smooth clutch-to-clutch shifting can be accomplished without requiring increased number of the valves in particular. However, in a case where the signal pressure is not supplied to the control valve due to a failure occurred in the aforementioned switching valve, the line pressure is directly applied to the brake in setting the second gear stage; thereby regulation of hydraulic pressure applied to the brake for the second speed gear stage cannot be performed, resulting in possibility of increased shift shock.
In addition, in case of the gear shifting from the third gear stage to the second gear stage which is so-called clutch-to-clutch shift for example, if the line pressure is applied to the brake for the second gear stage during this gear shifting, there occurs a so-called tie-up condition, wherein two brakes have unexpectedly torque capacity; causing an excessive braking torque in addition to aggravated shift shock and thereby possibilities of deterioration in durability of the friction material of the brake means due to its wear.
The primary object of the present invention is to provide a control system for automatic transmissions capable of prevention of the shift shock and deterioration in durability of the friction material and so on by sure and rapid detection of failure.
The another object of the present invention is to restrain abnormal torque acting on the frictional engagement means in a case of failure.
The further object of the present invention is to prevent the control system from accepting an erroneous signal as a datum.